Chemical Defense of Leaf Beetles (Dr. Antje Burse)

Introduction

Beetles have used plants as a food source for about 230 million years, which has contributed to reciprocal adaptation and the enormous biodiversity that is found today in both organism groups. Phytophagous species account for more than double the non-herbivorous taxa. This disparity became especially pronounced with the increasing diversity of angiosperms in the post-Cretaceous period. In response to herbivores, plants developed several morphological and biochemical adaptations which allowed them to wage a kind of chemical warfare; one strategy of this war was based on toxic secondary metabolite production, storage and eventually release. As some insects became adapted to these metabolites, interactions between the two organism groups occasionally led to highly specific relationships.

Beetle species manly feeding on green plant parts are combined in the taxon Phytophaga that includes Curculionoidea and Chrysomeloidea. The latter combine Cerambycidae and the Chrysomelidae sensu latu. In our projects we are focusing on the Chrysomelidae with special emphasis on the subtribe Chrysomelina where all developmental stages – from eggs to adults – are chemically protected against predators.

In certain cases the defensive compounds are stored in specialized structures of the body and used to repel predators, such as in the larvae. The latter possess nine pairs of defensive glands on the last two thoracic and first seven abdominal tergites. Each of the exocrine glands is composed of many secretory cells which are attached to a large reservoir. When the larvae are stimulated, they emit secretions from the tips of the glandular tubercles. As soon as the disturbance is over, the secretions are sucked back into the reservoir. The anti-predatory effect of the secretions can be attributed either to autogenously synthesized defensive compounds or to sequestered plant-derived glucosides converted within the reservoir by a few enzymatic reactions into deterrents.

Larva of Chrysomela populi with
droplets of secretion.

Defensive system
of Chrysomela populi.

According to phylogenetic analyses of Chrysomelina species by Termonia et al. (2001)1, archetype beetles are currently considered to produce their repulsive compounds de novo (e.g. iridoid monoterpenes), while more advanced species sequester and process aromatic plant constituents (e.g. salicin to salicylaldehyde) or use a mixed insect-plant route of synthesis (e. g. esterification of de novo synthesized butyric acids by alcohols retrieved from the plant).

A combination of the above-described strategies of allomone production with the host plant families mirrors the reciprocal adaptation of Chrysomelina beetles to their hosts. Species synthesizing the deterrents de novo feed on different plant families, such as Brassicaceae or Polygonaceae. In contrast, Chrysomelina members whose larvae sequester salicin are adapted exclusively to Salicaceae. Larvae of Chrysomela lapponica sequester a blend of glucosidically bound leaf alcohols. It is remarkable that this species has developed allopatric populations which colonize salicaceous and betulaceaous plants that effect the composition of the defensive secretion.


Maximum parsimony reconstruction of the evolution of Chrysomelina considering the synthesis of deterrents in the defensive glands of the larvae (A-C) and the host plant’s affiliations (D) (adapted from 1Termonia et al.2001, PNAS 98:3909)

Due to the different stages of host plant dependence of the chemical defense, the subtribe Chrysomelina represents an excellent model system to discover factors that interfere with co-evolution of insects and plants. To address these questions, the tight interplay of molecular, analytical and chemical techniques is essential for us.